High frequency apparatus



May`13, 1941- J. P. BLEwE'r-r' ETAL 2,241,976

HIGH FREQUENCY APPARATUS Filed April 25, 1940 3 Sheets-Sheet 1 Inventors: John P. Bl ewetl.,

Simon Rar-no,

en` Attorney.

May 13, 1941.

J. P. BLEWETT EIAL HIGH FREQUENCY APPARATUS Filed April 25, 1940 s sheets-sheet 2 EUI'IH Inventors:

John F. Blew/ett,

Simon Ramo,

Tl'welr` Attorh ey.

May 13, 1941- J. P. BLEWETT Erm. 2,241,975

HIGH FREQUENCY APPARATUS Filed April 25, 1940 5 Sheets-Sheet 3 Aftosa/MTM OUTPUT F1325. (j Fig. IO.

Inventors John P. Blewett Srnon Ramo,

Their Attorney.

Patented May 13, 1941 UNITED STATES` PATENT OFFICE man mEQUENcr APPARATUS John P. Blewett, Scotia, and Simon Ramo,

Schenectady. N. Y., mignon to General Electric Company, a corporation of New York Application April 25, 1940, Serial No. 331.562

The present invention relates to high frequency apparatus. l

It is an object of the invention to provide improved circuit means which are usable at ultrahigh frequencies and which are readily variable as to electrical characteristics so as to permit controlled variations of electrical energy stored or transmitted thereby.

This object is fulillled by the use of an electronic device of the magnetron type which is so arranged that its electrode elements form components of the concentric conductor transmission line or other wave-guiding structure suitable for use in the ultra-high frequency range. It is found that the properties of such a device may be varied in a manner calculated to cause a corresponding modification of the wave-propagating characteristics of the structure in which it is lncorporated, and that by this means one may readily accomplish variable or controllable eilects considered difllcult or impossible to obtain by systems heretofore available.

The features which we desire to protect herein are pointed out with particularity in the appended claims. The invention itself. together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in which Fig. 1 is a longitudinal sectional view of an apparatus suitably embodying the invention; Figs. 2 and 3 represent altersectional view of a structural modification of the (ci. 17a-171.5).

and I3 by asecond battery I8. A magnetic eld,

provided. for example, by coils 20 and 2|-, is pro duced axially of the'cathode II and serves to impart a rotary motion to electrons emitted cathode.

Due to the electron motion above referred to, a high frequency voltage is generated between the anode segments 2 and I3, the frequency of such voltage being determined principally by the characteristics of the circuit with which the anodes are externally connected.A In the present case, this circuit is of a type which is suitable for use at frequencies on the order of 100 megacycles or higher. It comprises an elongated inner conductor and a coaxial outer tubular conductor 24, these elements forming in combination a wave-guiding structure of the type frequently referred to as a coaxial conductor transmission line. Such structures are capable of propagating high frequency electromagnetic waves without substantial energy dissipation and are therefore useful either as high frequency tank circuits or as transmitting agencies for high frequency power. In the arrangement illustrated, the conductors 23 and 24 form parts of a resonant system which by the is short circuited at the extremity remote from invention; Figs..5 and 6 are enlarged fragmentary views, taken in different planes, oi' certain parts of the apparatus of Fig. 4; Figs. '1 and 8 respectively show additional structural variations of the invention; Fig. 9 shows the application of the invention in connection with a cavity reso-- nator which is used as a high frequency tank circuit; and Fig. 10 shows a somewhat similar application of the invention in connection with a cavity resonator used as a modulating device.

Referring particularly to Fig. 1, there is shown an embodiment of the invention suitable for use in a system of frequency modulation.

The illustrated arrangement includes as a source of high frequency electromagnetic waves a l the oscillation generator I0 by a conducting part 21. When appropriately excited, such a system tends to form standing waves whose wave length is determined by the consideration that the distance between the generator I 0 and the shortcircuiting part 21 must approximate some odd number of quarter waves.

In the present case, the operating wave length is made controllably variable by incorporating in the resonant structure a second magnetron device 29. This device includes as an anode a conducting cylinder 30 which is in such proximity to the tubular conductor 24 as to be closely coupled thereto and to form, in eilect, an integral part of the conductor. The cathode of the device29 consists of an elongated conductive element 3i which is electronemissive throughout substantially its entire length and which constitutes a colinear extension of the conductor 23. The emitting portion of the cathode is shown as being indirectly heated through the agency .of a resistance heater 32 and a heating battery 33. l'A choke coil 3,4 prevents circulation of high frequency currentsin the heatingv circuit.

The cathode 3| is connectedto the part 21 an is, therefore, effectively grounded at low frequencies. The anode, on the other hand, is maintained positive by means of a battery 31 so that it tends to attract electrons emitted from the cathode. In order to prevent such electrons from actually crossing the lnterelectrode space, they are deflected by means vof a uniform magnetic field directed axially of the cathode. This field is provided by the combination of a coil I9 and an energizing battery 4I! and acts to produce orbital motion of the electrons leaving the cathode 30. The result, therefore, is to create a rotating space charge in the region between the cathode and the anode 30.

It will be shown in the following that variations in the character of this space charge may be used tov control the wave-propagating characteristics of the structure formed by the conductors 23 and 24 and thus to modify the frequency of resonance and other operating properties of the structure. This control may be accomplished either by varying the magnetic field, as by adjustment of the battery 40, or by including a source of variable potential in circuit with the anode 30.

The ability of the device 29 to control the wavepropagating characteristics of the structure with which it is associated depends in large part upon the influence of the rotating space charge developed Within the device on the dielectric constant of the region which the space charge occupies. 'I'he dielectric constant of the chargelled region may be represented by the character K and is found to be variable with the applied magnetic eld H and the operating frequency w according to the following relationship:

e2H: 2mc2w2 (l) where e is the charge on an electron, m is its mass, and c is the velocity of light. Investigation of the variables of this equation shows that even.

' is so small in relation to capacitance variations obtainable by other means that the use of such a device'offers little'or no advantage below about 100 megacycles. On the other hand, in a high frequency system whose functioning is governed by the distributed constants of its parts, the performance vof the system may be critically responsive to small variations in the distributed capacitance of one of the system elements. For this reason, at frequencies in the range between one hundred megacycles and several thousand megacycles, an arrangement such as that illustr-ated in Fig. 1 possesses outstanding utility. A factor which is especially important in this connection is the circumstance that the operation of a device such as 29 is substantially free of objectionable transit time eects. As is well known, these eects tend to render electronic devices of more usual character inoperative or impractical when it is attempted to-extend their application to the ultra high frequency range.

In connection with an arrangement such as that of Fig. 1, the effect of adjusting the dielectric constant of the space charge within the device 29 is to modify the distributed capacitance of the portion of the wave-guiding structure which it employs, such portion becoming substantialiy non-capacitive when the dielectric constant is zero. The effect of th'ese considerations on the wave-propagating properties of the resonant structure as' a ,whole is determined in part Iby the location of the device 29 and will be maximum when the device is at a voltage loop of the standing wave pattern existing in the structure. Since the axial current at such a point is a minimum, the effect of the distributed series inductance may be neglected, and if distributed capacitance is also eliminated, the affected portion of the structure may be regarded as a region ofl zero impedance,. (the resistance being negligible). 'I'his is equivalent to thev complete elimination of a section of the structure and will have results generally similar to those which would follow moving the short-circuiting conductive part 21 closer to the generator Hl. That is to say, reducing or eliminating the shunt 'capacitance of the region subtended by the device 29 by reducing the dielectric constant of its space charge vwill decrease the resonant Wave length of the ciated.

Even with the dielectric constant held at some -flxed value-provided such value is appreciably different from unity-the distributed capacitance between the cathode 3| and the anode 30 may be varied by varying the physical extent of the space charge.v 'This may be done, for example, by changing the voltage impressed between' the anode and cathode. The attendant variation of the radius a of the region occupied by' the space charge is given by the following relationship:

V- 2 1 2i b i@ e -a( 0g /mcz Y (L) wherein V represents the impressed voltage, b is the radius of the anode, and the other quantities are those which have been previously identified in connection with Equation (l) Due to the relationship last referred to, it is readily feasible to operate the apparatus of Fig. 1 'as part of a system of frequency modulation. This may be done in one way by placing in the anode-cathode circuit of the device 29 a source 42 of variable voltage (e. g. an audio or video signal) with which it is desired to modulate the output of the high frequency` oscillator I0. Variations in the potential supplied by the source 42 will, in v accordance with the principles above described. produce corresponding variations in the character of the space charge in the device 29and consequently will change the distributed constants of a section of the resonant structure. The ultimate result will be to modify the equivalent length and tuning of the resonant structure and hence to change the frequency of the voltage developed by the device l0 in accordance with the signal pattern. The frequency modulated waves thus generated may be fed to an appropriate output system by any known means, such as an inductive loop 45 which is coupled to the space within the resonating structure and which is associated with va coaxial conductor transmission line having an inductive loop the output of this generator is fed into a wave-guiding structure generally similar to that described in connection with Fig. 1, the energy transfer being accomplished by placing the loop 5I in proximity to a second loop 52 which terminates the wave-guiding structure. 'I'he structure itself includes an outer conductor 53 and an inner conductor which consists at one end of a thin tubular part 54 and at the other end of a fllamentary extension 55.

Intermediate the ends of the structure there is provided a. magnetron device 55. lIhis encloses an electron-emissive cathode 51 which comprises a colinear extension of the conductor 54 and which is indirectly heated by current supplied to its heating .filament 51' from a battery 58 through a choke coil 59'. An anode 59 concentricaliy surounds the cathode 51 and is maintained positive with respect toit by means of a battery 60. As a result of this arrangement, waves generated by the source 59 and propagated axially along the conductors 59 and 54 are caused to pass through the region subtended by the device 59 before they can be" transmitted to the other extremity of the transmitting system. The waves so propagated are assumed to be fed to an appropri-ate output circuit as indicated by the legend appearing at the right hand end of Fig. 2.

In order to cause the signal received by the output circuit to be of amplitude modulated character.' it is proposed to produce a magnetic field in the vicinity of the device 59 of such value lthat the dielectric constant of .the space charge Within the device departs materially from unity. Such a field may be provided, for example, by means of a coil 5l which is energized in any desired manner (not illustrated). With the space charge properly adjusted, the potential difference between the cathode 51 and the anode 59 of the device is varied in accordance with a modulating signal derived from a source 64.

Under certain conditions of use the electrodes 51 and 59 may be regarded as electrically con tinuous parts of the conductors 54 and 55. However, to the extent that the dielectric constant of the region between the cathode 51 and the anode 59 departs from unity, such region must be regarded as a discontinuity in the wave-prop agating system. Moreover, the character of the discontinuity is necessarily variable in accordance with such variations ras occur in the potential impressed between the electrodes, at lea-st insofar as such variations tend to vary the space charge characteristics. For these reasons, the ability of the resonant structure as a whole to transmit wave energy of a particular frequency is found to be controllably variable in accordance wtih the signal variations of the voltage source 54. Assuming that the wave energy supplied to the system by the generator 50 is of constant magnitude, these changes in transmitting ability will manifest themselves as .an amplitude modulation of the signal received at the output circuit.

A still further application of the invention is reprented in Fig. 3, which shows its use as a means for matching the` impedances of two circuit elements between which it is desired to transfer power.

Itis known in this connection that the transfer of power between two electrical'. elements of different impedance characteristics may .be improved by connecting the elements through a device which has the effect of matching the impedance of one element Ito that of the other. It

has also been proposed to use transmission line sections as a means of accomplishing the impedance-matching' function. However, this expedient is diflicult of application for the reason that exact adjustment of the characteristics of the transmission line section which is to be used as the matching agency is often impracticable. Such adjustment is facilitated by an arrangement such as that of Fig. 3.

In the figure referred to, there is shown an arrangement for the transfer of power between a signal-receiving antenna 10 and' a device for amplifying the received signal. The latter device is not shown in the drawings, but is assumed to be appropriately connected to the illustrated structure as indicated Iby the legend at the right thereof.

As in the :arrangements previously described, the energy-transferring means includes an outer tubular member 1| and a coaxial inner member comprising an elongated hollow conductor 12. The antenna 10 is coupled to the space Within the member 1I by means of an inductive loop 19.

, Also within the conductor 1l, there is provided a magnetron device 15. This device includes an indirectly heated cathode 11 which is colinear with the conductor 12 and which is directly connected thereto -so as to form a conductive extension of lthe conductor. Heating current for the cathode is supplied through a direct current circuit which includes the combination of the tubular conductor 12 and another wire 18 insulatingly arranged within the hollow interior of the conductor 12. These conductors are terminally connected to a heating battery 90.

Surrounding lthe cathode 11 there is provided a .tubular anode 8l which is maintained at a xed positive potential with respect to the cathode by means of a battery 82. A magnetic field is produced axially of the magnetron device by means of a coil 83.

As has been previously explained, Ithe space charge conditions existing between the cathode 11 and the anode 8| determine to a significant degree the wave-propagating characteristics of the energy transfer system provided by conductors 1I and 12. Moreover, by controlling these characteristics, the impedance-matching action of the structure may be modified at will. Such adjustment may be accomplished, for example, by means of a variable voltage source associated with the coil 83. vIn the intended use of the apparatus, the potential provided by this battery is adjusted until the characteristics of the resonant structure as a whole are such as to cause most satisfactory transfer of .power between the antenna 10 and the amplifier to which it is connected.

It should be understood that the particular means of obtaining a rotating space charge which has been described in connection with Figs. 1to3is not essential to the purposes of the invention, and in Fig. 4 there is shown an alternative arrangement useful for this same purpose. In this case, the structure shown may be assumed to lconstitute the extremity of a modulating system such as that which is illustrated in Fig. 1. It includes a tubular conductor having .a lamentary conductor 9| running axially thereof. At an intermediate region, a section 9| of the conductor 9| is enclosed in an evacuated glass envelope indicated at 93. The enclosed section of the conductor is further surrounded by a tubular conductive member 94 which, in this case.- is maintained atthe same D. C. potential as the conductor.

4 Electrons are projected into the space between the conductor section 9|' and the tubular memlber 94 by the use of a form o-f electron gun which is illustrated in/detail in Figs. 5l and 6. Thisy spect to such member.by a battery |04 so that elctrons emitted by the cathode are subjected to an accelerating field. An opening |03 provided in one Wall of the reentrant chamber |02 permits at least some of the electrons generated by the cathode to be projected into the main space enclosed by the member 94.

Within such space and adjacent the opening |03 there is provided an additional electrode |05 (Fig. 6) which is biased negatively with respect to the member 94 so that it tends to repel electrons coming through the opening |03 and thus to give the electrons a component of motion in :a direction parallel -to the axis of the envelope 93. In'addition, a component of rotary motion is imparted to the electrons by the action of lthe magnetic field-producing structure' 95 so that the electrons are caused to follow a more or less helical path as indicated by the dotted line A of Fig. 4. In following this path, the electrons move orbitally about the conductor 9 as an axis of gyration so that their aggregate effect is that of a rotating space charge comparable to the space charge created by other means in the de? vices of Figs. 1 to 3.

In order to control the character of this space charge and thus to modify the wave-progagating characteristics of the structure withwhich the device 93 is associated, a control element |01 in the nature of a control grid may be provided in connection with the cathode 91 (see Fig. 5). This element, which is shown as a circular ring, is maintained at a negative average potential with respect to the cathode 91 and is connected to a variable voltage source |09. Variations of this source tend to change thenumber of electrons projected into the space enclosed by the tubular member 94 and thus to vary the space charge within such member. These variations result in corresponding changes in the reaction of the device 93 on the resonant properties of the system provided by the conductors 90 and 9| and may, therefore, be used to produce modulation or other variable effects.

Another structural embodiment of the variable space charge device is shown in Fig. 7, which illustrates the device apart from the wave-guiding structure with which it is intended to be used. In general, the construction of the device is similar to that described in connection with Fig. 4 in that it includes a conductor ||2 extending through ,the dev'ce and an electron gun ||3 for tained at a common D. C. potential by being grounded through choke coils ||8 and ||9.

In use, the conductors ||2 and ||0 comprise portions of a parallel wire transmission line whose wave-propagating characteristics it is desired to control. As in the device of Fig. 4, such control may be accomplished by varying the space charge between the conductors by changing the number of electrons projected from the electron source.

A still further structural modification is shown in Fig. 8, which illustrates a closed envelope |2| within which is provided a pair of parallel platelike electrodes |22 and |23. The electrode |23 is assumed to'be of electron emissive character and is heated indirectly by means of a resistance heater |25. A difference of potential is maintained between the electrodes |22 and |23 by appropriately connecting them to a voltage supply source |28 through choke -coils |29 and |30. A magnetic eld provided by a coil |26 serves to assure limited motion of electrons emitted by the member 23 and to produce a space charge in the region between that electrode and the electrode |22; such space charge being variable either by adjustment of the magnetic field or by changing the interelectrode potential. In the use of the device, the electrodes |22 and |23 serve asA components of` a parallel wire transmission line, the conductors of which are respectively indicated at |3| and |32. The wave-propagating properties of the line may be modified by varying the character of the space charge in the device |2|.

Another application of the invention is shown in Fig. 9 which illustrates its use in connection with a cavity resonator employed as a high frequency tankcircuit. As indicated by the legend near the top of the figure, the tankv circuit is assumed to serve a high frequency oscillator which is coupled into` the resonator by an indue.. tive loop |34.

The resonator itself consists of a hollow spherical metal body |35. It is known that under proper circumstances such a body may be made to resonate electrically by yi'rtue of the propagation of` electromagnetic waves within its interior and that in view of such resonance, it vmay serve as a circuit element vfor frequencies in the ultra-high frequency range.

The resonantfrequency of a structure such as that described is a function of the physical dimensions of the structure and depends upon the distributed electrical constants of the yparts which make up the structure. In the present case, provision is made for varying the natural operating frequency of the structure |35 by coupling it to a second wave responsive system which is itself of controllable character. -Such a system is shown as comprising a conductive tubular part |36 which projects outwardly from theresonator |35 and which'encloses a magnetron device |31 of the ,type described in connection with Fig. l. To avoid complication of the drawings, the direct current electrical connections to the electrode elements of the device are omitted, but it is assumed that they include means .for supplyingv heating current to the cathode |38 and for maintaining a suiiicient diiierence of potential between saild cathode and the,surrounding anode |39 to produce an appreciable space charge in the interelectrode region. Electrons are pre- 1 vented from ilowing directly to the anode by means-of a coil |45 arranged c o-axially with the cathode.

The cathode |38is coupled to the region ericlosed by the hollow bodyl|35 by means of an inductive loop |4| connected bewteen the cathode and the body. Consequently, wave disturbances or oscillations produced within the body tend also to lexcite the auxiliary resonant structure provided by the combination of the tubular extension |36 and the cathode |38. It will be readily understood that the wave-propagating characteristics of this auxiliary structure may be controlled by varying the space charge in the region between the cathode |38 and the anode |39, in accordance with principles previously set forth. Such control may be accomplished in one Way by means of the coil |45 by connecting the same with a variable voltage source as indicated at |46.

Variations produced in this way will produce a corresponding change in the nature of the reaction of the auxiliary resonant structure |36 on the wave-propagating characteristics of the hol low body |35 and will consequently tend to modify its natural resonant frequency. By this -lmeans the resonant characteristics of the body may be adjusted to secure optimum cooperation with the oscillation generator with which the body is assumed to be connected.

Fig. represents the application in a modulating system of an arrangement similar to that of Fig. 9. In this case, it is proposed to effect amplitude modulation by varying the transfer of wave energy between an input system connected to an inductive loop |50 and an output system connected to a second loop |5|. To this end both loops are coupled to the space enclosed by a hollow cavity resonator |53. The Wavepropagating characteristics of the resonator are controlled by means of a magnetron device |31" which is enclosed within an extended portion |36 of the cavity |53. (Inasmuch as the device |31' and its associated parts are identical with the similar elements shown in Fig. 9, they are similarly numbered.) By varying the potential applied between the cathode |38 and anode |39', as by connecting these elements to a source of signal voltage, a corresponding modification of the wave-propagating characteristics of the cavity resonator as a whole may be obtained. If a signal of constant intensity is applied to the input loop |50, this modification will manifest itself as an amplitude modulation of the signal received at the loop |5|.

While we have described our invention in connection with particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled inthe art without departing from the invention. We therefore. aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In an ultra-high frequency system, a source of high frequency electromagnetic waves. a structure lfor propagating waves derived from said source and Having wave-propagating characteristics which are fixed mainly by the distributed constants of its component parts, an evacuated electronic device forming part of the said structure and having spaced electrodes the distributed constants of which determine to a significant degree the wave-propagating characteristics of the structure, means including a magnetic field-producing agency and a source of potential connected between the electrodes for maintaining in the space between them a rotating space charge which affects the distributed capacitance of the electrodes by its presence without producing appreciable current flow be tween the electrodes, the said space charge being variable in response to variations of the magnetic field and of the said potential, and means for controllably varying one of the lastnamed quantities in order correspondingly to modify the wave-propagating characteristics of the structure.

2. In combination, a source of high frequency electromagnetic waves, a pair of elongated parallel conducting members along which waves from said source are propagated, the wave-propagating characteristics of the members being determined mainly by their distributed electrical constants, an evacuated electronic device including spaced conducting elements which, in effect. constitute parts of the said elongated members. means for producing between the said elements a rotating electronic space charge which affects the distributed constants of the elements by its presence without producing appreciable current flow between the elements, and means for varying the character of the said space charge in order correspondingly to modify the wave-propagating characteristics of the said elongated members.

3. In combination, a source of high frequency electromagnetic waves, a hollow conductive body which functions as a wave-propagating structure for waves derived from said source, an evacuated electronic device within the said body and including a pair of spaced conductive elements the distributed constants of which determine to a significant degree the wave-propagating characteristics of the body, the device having means for producing between the said spaced elements a rotating electronic space charge which affects the distributed constants of the elements by its presence without produc-- ing Iappreciable current flow between the elements, and means for varying the character of the said space charge in order correspondingly to modify the Wave-propagating characteristics of the said hollow body 4. In combination, a source of ultra high frequency electromagnetic waves,l a structure which functions as a wave-guide for electromagnetic waves propagated from said source and which comprises an elongated inner conductor, a tubular outer conductor coaxial with said inner 4con-- ductor, an evacuated electronic device including an elongated conductive element serving as a part of the said inner conductor and a coaxially arranged tubular element forming, in effect, a part of the said outer conductor, means for producing a rotating electronic space charge in the region bounded by the said elements without producing appreciable current flowbetween the elements, and means for Varying the character of the said space charge in order correspondingly to modify the Wave-propagating characteristics of the structure as a whole.

5. In combination, a source of high frequency electromagnetic waves, a pair of elongated parallel conductors along which waves from said source are propagated, an evacuated electronic idevice including an elongated linear cathode which serves as a part of'one of said conductors and which is electron-emissive throughout a substantial portion of its length, an anode parallel to said cathode and forming, in effect, a part of the other of-said conductors, means for producing an electric field between the said anode and cathode so as to establish a space charge in the region between them; meanslor producing a magnetic field -eiective to cause continuous rotation of the said space charge about the said cathode and thus t0 prevent appreciable current flow between the anode and cathode, and means for varying at least one of the said eldsin order to modify the characteristics of the space charge and correspondingly to change the wave-propagating characteristics of the said conductors.

6. In combination, a source of high frequency 'electromagnetic waves, a structure which functions as a Wave-guide for waves derived from said source and which comprises an elongated inner conductor, a tubular outer conductor coaxial with said inner conductor, an evacuated electronic device including an elongated cathode which serves as a part lof the said inner conductor and a tubular anode surrounding the cathode and forming in eiect a part of the said outer conductor, means for producing an electric eld between the anode and cathode so as to establish a space charge between them, means for producing a magnetic field effective to cause continuous rotation of the said space charge about the cathode and thus to'prevent any appreciable current ow between the Ianode and cathode, and means for varying one of the said fields in order. to modify the characteristics of the said space charge and correspondingly to change the wave-propagating characteristics of the structure as a whole.

7. In a high frequency system, a wave-guiding structure having wave-propagating characteristics which are xed mainly by the distributed constants ofv its component parts, an evacuated electronic device forming part of the said structure and including a pair of elongated parallel conductive elements the distributed constants of which determine to a significant degree the wavepropagating characteristics of the structure, means for producing a stream of electrons which moves ralong a helical path coaxial with one of the said conductive elements and which affects the distributed capacitance of the elements by its presence without causing appreciable current flow between the said elements, and means for varying the said stream in order correspondingly to modify the wave-propagating characteristics of the said structure.

JOHN P. BLEWETT.

SIMON RAMO. 

